GB1587727A - Removal of impurities from xylene mixture by distillation - Google Patents

Description

(54) REMOVAL OF IMPURITIES FROM XYLENE MIXTURES BY DISTILLATION (71) We, IMPERIAL CHEMICAL INDUS TRIES LIMITED, Imperial Chemical House, Millbank, London SW1P 3JF, a British Company, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to the removal of impurities present in mixtures containing one or more xylene isomers.

Many processes for the manufacture of xylenes are now in commercial operation.

Such processes frequently manufacture a mixture of two or more of the xylene isomers (and sometimes ethylbenzene also) with the result that separation processes must be employed in order to isolate the desired isomer or isomers. Paraxylene is usually tie most desired isomer processes have been devised to separate this isomer from orthoxylene, metal-xylene and ethylbenzene. In addition, isomerisation processes have been developed to convert the unwanted isomers into more para-xylene.

Separation of the three individual xylene isomers from mixtures containing them has certain difficulties. The boiling points of the three isomers are so close together that separation of the isomers by distillation is extremely difficult. Ortho-xylene may be separated from its mixture with one or both of the other isomers provided that the distil lotion is carried out carefully and precisely.

However, meta and para-xylene have virtually the same boiling point as one another and fractional distillation cannot be used to separate them.

Fractional crystallisation is another method that has been proposed and used to separate the xylene isomers and fractional crystallisation methods work quite well.

Para-xylene crystals of high purity can be obtained in these methods and the mother liquor is frequently returned to an isomerisation process for conversion of at least some of the o-xylene, m-xylene and ethylbenzene in it to more para-xylene. During isomerisation, small amounts of impurities are also formed. These impurities can be both lighter, for example benzene and toluene, than the xylenes and heavier, for example trimethylbeuzene. It is desirable that these impurities should be removed from the xylenes mixture before the mixture is subjected to further processing, for example fractional crystallisation, to separate the desired xylene isomer. It is customary to remove the impurities in a distillation operation involving the use of two sills. In this operation, a vapour feed from the isomeriser is fed to the first still in which most of the heavier impurities are removed as bottoms.

The xylenes together with the lighter impurities are removed over head and are fed to the second still. Normally, the feed to the second still is supplemented with a minor amount of fresh feed. The second still is operated so that the light impurities are removed overhead and the now much-purified xylenes stream is removed as bottoms.

According to the present invention a process for the separation of lighter and heavier hydrocarbon impurities from a mixture thereof with one or more xylene isomers comprises subjecting the mixture to a single distillation operation to produce a bottoms stream comprising the heavier impurities, an overhead stream comprising the lighter impurities, and a side-stream comprising one or more xylene isomers wherein the bottoms stream and overhead stream each comprises less than 10% by volume of xylenes and the side stream comprises at least 95% by volume of xylenes.

Preferably the bottoms stream and overhead stream each comprises less than 5% by volume of xylenes, more preferably less than 2% and the side stream preferably comprises at least 97% by volume of xylenes, more preferably at least 98%.

A preferred embodiment of the present invention is hereinafter described with reference to the accompanying drawing which is a schematic diagram of a distillation column.

EXAMPLE Feedstock for distillation enters the distillation column 1 along line 2 from a xylenes isomeriser (not shown). Lighter impurities are removed overhead by line 3 and heavier impurities iare removed as bottoms by line 4. Purified xylene product is removed along line 5 to a separation unit (not shown), for example a fractional crystalliser, to separate the individual xylene isomers.

In operation, a mixed xylenes feed of composition 0.6% benzene, 2.3% toluene, 93.7% C8 aromatics and 3.4% C8 aromatics was fed along line 2 to the column 1. Heavy ends of composition 2.5% C8 aromatics, 97.5% C8 aromatics were removed along line 4 and a stream of lighter impurities of composition 30.3% benzene, 63.3% toluene and 6.1% C8 aromatics was removed along line 3. The xylenes product stream of composition 1.1% toluene, 97.8% C8 aromatics and 1.1% C8 aromatics was removed along line 5. (All percentages are by weight.) The process of tills invention produces a very pure xylenes stream using only one distillation column instead of the two columns which had hitherto been thought to be necessary. This allows not only appreciable savings in the capital cost of a new xylenes plant but also considerable savings in operation by virtue of a much lower energy requirement.

WHAT WE CLAIM IS:- 1. A process for the separation of lighter and heavier hydrocarbon impurities from a mixture thereof with one or more xylene isomers which comprises subjecting the mixture to a single distillation operation to produce a bottoms stream comprising the heavier impurities, an overhead stream comprising the lighter impurities, and a sidestream comprising one or more xylene isomers wherein the side stream comprises at least 95% by volume of xylenes and the bottoms stream and overhead stream each comprises less than 10% by volume of xylenes.

2. A process as claimed in claim 1 in which the side stream comprises at least 97% by volume of xylenes and the bottoms stream and overhead stream each comprises less than 5% by volume of xylenes.

3. A process as claimed in claim 1 or 2 'in which the side stream comprises at least 98% by volume of xylenes and the bottom stream and side stream each comprises less than 2% by volume of xylenes.

4. A process for the separation of lighter and heavier hydrocarbon impurities from a mixture thereof with one or more xylene isomers substantially as hereinbefore des cribed with reference to the Example.

5. Xylene whenever produced by a pro cess as claimed in any one of the preceding

Claims (1)

1. claims.